This invention generally relates to a method of controlling movement of a robot. More particularly, this invention relates to a method of controlling the movement of a tool supported on a robot wrist by moving only the robot wrist.
The industrial use of robots has been a growing industry for a number of years. Robots are being used for a larger variety of applications than ever before. Each new application for a robot presents special challenges and difficulties that must be overcome to provide efficient operation while performing the desired task.
One example application that is particularly challenging is where a robot must repeatedly move in a back-and-forth pattern to perform a desired operation. One example operation that requires such motion is known as a weaving-like motion, which is typically used during a welding process. Several attempts have been made at developing a methodology for controlling a robot to perform a weaving-like welding operation.
One example of a proposed solution is found in U.S. Pat. No. 4,150,329. The disclosure of that patent suggests a technique for tracing a 3-dimensional weaving-like pattern that is defined by relative amounts of movement along three perpendicular reference axes. A major shortcoming of that proposal, however, is that when a principal welding line is not in a specific orientation relative to one of the reference axes, the weaving-like pattern becomes abnormal and unsatisfactory results are achieved.
Another example proposed solution is found in U.S. Pat. No. 4,621,333. While the arrangement of that patent may represent an improvement over the previously mentioned patent, it does not provide a workable solution in many instances. One drawback associated with the latter patent is that it requires teaching a plurality of welding points to the robot that later will be followed during operation. Such teaching operations can be undesirably complex, time-consuming and, moreover, may not be as accurate as required in many instances.
A major shortcoming of all prior attempts is that they require moving a substantial portion of the entire robot arm to achieve the weaving-like pattern. In many applications, high frequency motion is desired and in some it is required. When a substantial portion of the robot arm must move to achieve the desired weaving pattern, large amounts of inertia must be compensated for because of the mass of the portions of the robot involved in the desired motion. In many instances, the inertia cannot be compensated for in a satisfactory manner. Therefore, the frequency of a back-and-forth motion is typically limited below desirable levels.
This invention overcomes the shortcomings and drawbacks of the prior art by providing a method for controlling the movement of a robot by only moving the robot wrist to obtain the necessary back-and-forth motion. With this invention, much higher frequencies can be obtained and most of the prior art difficulties in compensating for inertia and robot vibration are eliminated. Further, this invention provides a more easily implemented solution that is useful for a variety of applications while providing greater accuracy.